Method of cathodic protection of tanks filled with sea-water using an applied voltage and means for execution of the method



March 21, 1961 RlsBERG 2,976,226

METHOD OF CATHODIC PROTECTION OF TANKS FILLED WITH SEA-WATER USING AN APPLIED VOLTAGE AND MEANS FOR EXECUTION OF THE METHOD Filed July 5, 1957 2 Sheets-Sheet I 1 I l if I I I j: I 3 l is 1 I l l I 5 i j 1 j INVENTOR.

/L 2/5 BERG ATTOR EX March 21, 1961 E. RISBERG METHOD OF CATHODIC PROTECTION OF TANKS FILLED WITH SEA-WATER USING AN APPLIED VOLTAGE AND MEANS FOR EXECUTION OF THE METHOD 2 Sheets-Sheet 2 Filed July 5, 1957 H II l l l I ll INVENTOR. f/L/F 4 /5295 ATT R Y United States Patent METHOD OF CATHODIC PROTECTION OF TANKS FILLED WITH SEA-WAT ER USING AN APPLIED VOLTAGE AND MEANS FOR EXECUTION OF THE METHOD Eilif Risberg, Sentralinstitutt for industriell Forskning, Forskningsveien 1, Blindern, Oslo, Norway Filed July 5, 1957, Ser. No. 670,259

"Claims priority, application Norway July 5, 1956 3 Claims. c1.204-147 At present the mostly used and most efiicient method for protecting ballast tanks in tank ships is cathodic protection by means of anodes of magnesium, which are located at diiferent positions in the tanks in direct metallic contact with the steel.

This system is expensive, and a number of technical disadvantages are involved. The mounting of the anodes is time consuming and involves heavy work. After mounting of the anodes the eifect of same can not be adjusted at will. The result may be a considerable development of hydrogen on the steel in addition to undesirable quantities of lime-like layers which later will form flakes. Further, the magnesium metal represents a considerable danger in that sparks will be formed if an anode for one reason or another should become loose and knock against rusted iron.

It is evident that a system for cathodic protection of ballast tanks using an applied voltage, may be made far more flexible than the magnesium system, inter alia, with regard to adjustment of the current consumption. For such a system it is possible to choose between soluble anode materials as iron or aluminium or insoluble anodes as anodes of graphite or silicium iron but in both cases undesirable anode products will be created.

Both iron and aluminium, dissolved anodically will sooner or later precipitate in the ballast as hydroxides or similar compounds. The iron will, with a sufiicient oxygen content in the water form ordinary red rust. With deficiency of oxygen, for example during extensive periods of ballast, there will, however, be formed a black mud which later with admission of air partly may be converted to rust, but the mud will be black to the extent of the formation of magnetite (Fe O Aluminium will on the other hand form a white mud and partly also some greyish precipitation, due to the fact that small particles of metal will loosen from the anode. These anode materials are not in any way ideal, and it is evident that one, having the disadvantages in view, should try to eliminate any formation of mud in the tanks.

When using insoluble or inert anodes other products will be formed, namely chlorine and to a smaller extent oxygen. It is also evident that these products are undesired in the tanks.

The part the oxygen plays in the process of corrosion does not need any further explanation.

Chlorine is expected to have a number of undesirable effects in the tanks. Free chlorine has a strong corrosive action on iron and steel in the presence of water and is very poisonous and explosive mixed with hydrogen which can not be completely eliminated in the cathodic protection of such system. Chlorine/ hydrogen mixtures are within certain limits of concentration also sensitive to light. Chlorine is in addition very soluble in oils, and it may be expected, to a certain extent, to be retained in possible oil remnants in the tanks.

These effects will, however, not be equally pronounced in the tanks under ordinary conditions. Thus, the danger ICC of explosions during ballast trips of short duration will presumably be very small due to the large solubility of chlorine in water. It is to be expected that the poisonousness and the corrosivity will be of far greater importance" ind more pronounced the longer the ballast period will The object of the present invention is to eliminate these disadvantages by removing the anode products before theie products come into contact with the walls of the tan s.

The invention thus consists in a method for cathodic protection of tanks filled with sea water, using an applied volt-age, and the invention is mainly characterised in that the anode products completely or partly are removed from the system due to the feature that liquid in the area around or at the anode continuously or intermittently is removed from the tank.

An anode which is particularly suited for bringing the method into effect is mainly characterised in that the electrochemically active part of the anode is provided with a centrally located cavity communicating with the exterior of the anode through narrow channels or slots, which permit the sea Water, subjected to a pressure difference, to flow into the cavity from which the water maybe pumped out through a conduit and away from the tank, and another feature of the invention consists in that the anode may be provided with a protective cover which encloses the anode tightly when the anode is not to be used.

Other characterising features and details of the present invention will be evident from the following description with reference to the drawings, in which:

Fig. l is a diagramm-atical, very simplified view of a section through a part of a tank provided with means for cathodic protection in accordance with the present invention. For the purpose of illustrating the invention the dimensional relation between the tank and the means in accordance with the invention has been distorted,

Fig. 2 is also a diagrammatical view of an anode with a protective cover or sleeve, and

'Fig. 3 is a view of another embodiment of an anode with protective device.

*In Figure l, 1 indicates a part of a tank wherein there is suspended an anode 2 which in the example shown is assembled from a number of annular sections 3 of graphite, stacked on top of each other inside a tightfitting perforated sleeve 4 of electrically insulating and non-corroding material, as for example hard polyvinylchloride. Inside the stack of the section. 3 of graphite there will be formed a longitudinal cylindrical cavity 5' which at the lower end is closed by a lid 6 and which at its upper end by means of a conduit 7 is in communication with a pump 8. The conduit 7 extends to the bottom of the cylindrical cavity 5 and the pump 8 is designedto remove the Water which through the perforations 9 in the outer sleeve seeps between the graphite sections 3 and collects at the bottom of the cylindrical cavity. The cavity 5 may be brought into communication with the ambient atmosphere or if desired with a vacuum source by means of a conduit 10.

The necessary voltage is applied by means of an electric conductor 11 preferably to the lower of the graphite. sections 3 which on the drawing 'is referred to as 3'..

vacuum created in the cavity 5 bymeans of a vacuum,

'Patented Mar. '21, 1961 source, not shown, through the conduit 10, causes the water to flow in between the graphite section 3 and the water will be collected in the cavity 5 from which the water, as mentioned above,'will be pumped away by means of the pump 8 and the conduit 7. The anode products formed on the electrochemically active faces of the anode will thereby be removed before they have any opportunity of exerting any detrimental elfects on the walls of the tank, and before they have any opportunity of creating dangers, e.g. danger of explosions.

During all ballast periods of longer duration the tank should be replenished with sea water to replace the water which has been pumped out. The replenishing sea water will as a rule be saturated with oxygen. The water should therefore be led into the tank close to the anode to prevent the oxygen contained in the water from being distributed in the ballast, the concentration of oxygen in a tank filled with sea water normally decreasing with the time as a result of chemical and biological processes in the system. As indicated on the drawing the water may be distributed around theanode from spouts or nozzles 13 through a conduit 12. The pump feeding this conduit may with advantage be combined with the previously mentioned pump 8 as a double pump, so that the water which is fed through the conduit 12, in volume corresponds to the volume of the water which is pumped out from the cavity 5 of the anode through the conduit 7. To prevent the conduit 7 from pumping air when the water level at the bottom of the cavity 5 is sufficiently low, the conduit may at its lower end be provided with a float valve (not shown) which closes the conduit when no water is pumped.

Another method of maintaining a constant water level in the tank may consist in that the supply of water through the conduit 12 is controlled by a float operated valve (not shown on the drawing), the float of which being preferably positioned in the hatch coaming or in a similarly upwardly extending structure of the tank, as the tank should be filled up to the deck to obtain the best possible result of the protective system.

To prevent particles suspended in the ballast from affecting the anode this may be provided with a diaphragm or filter 14 and if required, the anode may be supplied with sea water inside this diaphragm by means of the conduit 12 with nozzles 13. 7

After having removed the sea water from the tank and when for example oil is to be loaded, the anode with attached conduits may be removed, but if the anode is to be permanently mounted in the tank the anode should be covered when the tank is filled with oil to eliminate any adverse effects of the oil on the anode. One design for such an anode is shown in Figure 2, and closing and opening of the anode will in this embodiment be performed automatically. The anode proper is also in this case referred to as 2, and the different conduits for supplying and removing water and the current carrying conductor as shown at 7, 10, 11, 12 are brought together and inserted in a tube 15. The electrode 2 is in this case covered by a sleeve 16 which is displaceable on the tube 15, and the sleeve 16 is provided with a float chamber 17, the size of which being so designed that the sleeve does not rise in oil of a density which is lower that the density of water.

When now oil is filled into the tank, the sleeve 16 will cover the anode 2 as shown in full lines on Figure 2, while the float chamber 17 when the tank is filled with water will have suificient buoyancy to lift the sleeve 16 to the position shown in dotted lines, and the anode will then be free to function as it is designed to, in the cathodic protection of the tanks.

When the anode is permanently mounted in a tank it may be advantageous to secure the anode by means of a stay rod 18 or by similar means.

If the tank 1 is to be filled with a heavier oil the density of which being similar to the density of water, the arrangement shown in Figure 2 will not be satisfactory, and Figure 3 shows an embodiment in which due attention is paid to this condition.

The protective sleeve 16 is also in this case provided with a float chamber 17 positioned inside the sleeve. In the side wall of the sleeve 16 there is provided a valve 19 which is electrically controlled by means of a coil 20. At the upper end of the sleeve 16 there is positioned an electrode 21 for example of magnesium and the electrode is insulated from the sleeve 16 which may form the second electrode in a galvanic element. The electrode 21 is through a conductor 22 connected to the coil 20, and the coil is further connected to the other electrode, which, as mentioned above is constituted by the sleeve 16.

When the tank is filled with heavy oil Which is insulating, no current will flow in the coil 20, and the valve 19 will be closed whereby the protective sleeve 16 will have the position shown in Figure 3, and will cover the anode 2.

After the oil has been pumped out, and when the tank is filled with sea water, the sea water will act as an electrolyte between the electrode 21 and the sleeve 16, and a current will flow in the conductor 22 and through the coil 20, so that the valve 19 will open and sea water will flow into the sleeve 16. The external and internal pressure on the sleeve 16 will then compensate each other, and the float chamber 17 will lift the protective sleeve 16 upwards along the tube 15 which in the same manner as in Figure 2, contains the necessary conduits and conductors.

In the upper position the electrode 21 on the protective sleeve 16 will be covered by a cap 23 which also is positioned on the tube 15, and which will prevent unnecessary consumption of the electrode 21.

When the sea water is again removed the protective cover 16 will fall down, and cover the electrode 2.

With the method in accordance with the present invention, for cathodic protection of tanks, there is thus obtained a method for removing the detrimental anode products before these products have an opportunity of acting on the walls of the tank and other structures, and with the different means for execution of the method there is inter alia provided anodes which may be permanently mounted and which will be protected against detrimental actions from oil, when the anodes are not in operation. The examples shown will only serve to illustrate the invention, and do not constitute any limitation for same as it, of course, is possible to use any kind of anode one should wish as long as the anode products are removed and carried out of the tank, without departing from the scope of the invention.

A number of other embodiments for the protective devices for the anodes are also possible, for example, the protective device for the anodes may be operated by means of air under pressure, but such embodiments will be obvious to persons skilled in the art after such persons have been informed of the idea forming the basis for the present invention.

I claim:

1. The method of protecting tanks filled with sea water against corrosion, which comprises positioning within the tank, a hollow insoluble anode having a wall formed for the passage of liquid therethrough, applying a voltage to said anode to cause a current to flow between said anode and the interior surface of the tank, the tank being the cathode, and to create anode products in a zone around said anode, reducing the pressure within the hollow of said anode with respect to the pressure in the sea water surrounding said anode to cause liquid to be drawn through the wall of said anode and removing said anode products from said zone around said anode by carrying said products through said wall by said liquid drawn therethrough.

2. The method as in claim 1 and controlling said flow of liquid through said wall by varying the reduction of pressure within said anode.

3. The method as in claim 1 and withdrawing said liquid drawn through the wall of said anode from the lower part of said hollow.

References Cited in the file of this patent UNITED STATES PATENTS Heise a a1. Feb. 17, 1942 Schumacher et a1. Sept. 19, 1944 Wesly Apr. 18, 1950 Higgins et a1. Oct. =18, 1955 FOREIGN PATENTS France Nov. 9, 1955 England Nov. 24, 1921 England Ian. 5, 192,2 

1. THE METHOD OF PROTECTING TANKS FILLED WITH SEA WATER AGAINST CORROSION, WHICH COMPRISES POSITIONING WITHIN THE TANK, A HOLLOW INSOLUBLE ANODE HAVING A WALL FROMED FOR THE PASSAGE OF LIQUID THERETHROUGH, APPLYING A VOLTAGE TO SAID ANODE TO CAUSE A CURRENT TO FLOW BETWEEN SAID ANODE AND THE INTERIOR SURFACE OF THE TANK, THE TANK BEING THE CATHODE, AND TO CREATE ANODE PRODUCTS IN A ZONE AROUND SAID ANODE, REDUCING THE PRESSURE WITHIN THE HOLLOW OF SAID ANODE WITH RESPECT TO THE PRESSURE IN THE SEA WATER SURROUNDING SAID ANODE TO CAUSE LIQUID TO BE DRAWN THROUGH THE WALL OF SAID ANODE AND REMOVING SAID ANODE PRODUCTS FROM SAID ZONE AROUND SAID ANODE BY CARRYING SAID PRODUCTS THROUGH SAID WALL BY SAID LIQUID DRAWN THERETHROUGH. 